1. Field
The present disclosure relates to the field of communications, and in particular to a nonlinear compensation method and apparatus and a system in a multicarrier optical communication system.
2. Description of the Related Art
A multicarrier communication system is a communication system based on multicarrier modulation, which is widely used in wireless communication and wired access networks due to its advantages, such as high transmission rate, high spectral efficiency, and anti-multipath frequency domain fading, etc. In a short-haul optical communication application, much attention is paid to a multicarrier communication system, especially a discrete multi-tone (DMT) system based on strength modulation and direct detection due to its simple structure and high transmission rate, which is deemed as one of the leading technologies in short-haul optical communication application scenarios, such as a data center of a next generation (Reference 1). However, in experiments, the performance of the DMT optical communication is constrained by nonlinearity of devices in the system. In order to achieve needed performance, such as 100 Gb/s, the nonlinearity of existing devices must be compensated (Reference 2). In the known art, compensation of nonlinearity is usually achieved by expansion of a Volterra radix, an order of the Volterra radix determining the order of the nonlinearity they may characterize or compensate. A simplest 2th order Volterra radix may be expressed as:
      y    ⁡          (      n      )        =            ∑              j        =        0            n        ⁢                  ∑                  i          =          0                n            ⁢                                    h            2                    ⁡                      (                                          p                i                            ,                              p                j                                      )                          ⁢                  x          ⁡                      (                          n              -                              p                i                                      )                          ⁢                              x            ⁡                          (                              n                -                                  p                  j                                            )                                .                    
When this formula is used for nonlinear compensation based on digital signal processing (DSP), h2 is a tap coefficient of a 2th order filter used for compensation, pi and pj time flags of the digital signal, and a time range n of pi and pj denotes a memory length of the compensated nonlinearity.
It can be seen from this example that in the known art, power consumption of the filter for nonlinear compensation exponentially rises along with the memory length of the nonlinearity it compensates. Hence, a method usually used currently is taking a very small numerical value for n, such as n=3, so as to control the power consumption of the nonlinear compensation. Such a method is feasible when a signal bandwidth is relatively small, for example, a bandwidth of a channel in wireless communication is at a magnitude of 10 MHz. While for high-speed optical communication, as a bandwidth of a signal is at a magnitude of decades of GHz, a nonlinear memory effect of a single device is enhanced, and at the same time, a cascade of linear effect and nonlinear effect between the devices greatly increases the memory length of the nonlinearity of the system. Therefore, for a high-speed optical communication system, there is an assumption that a very short nonlinear memory length performs nonlinear compensation is unable to achieve an expected effect.    Reference 1: Yutaka Kai et al, “Experimental Comparison of Pulse Amplitude Modulation (PAM) and Discrete Multi-tone (DMT) for Short-Reach 400-Gbps Data Communication”, thesis No.: Th.1.F.3, ECOC2013; and    Reference 2: Weizhen Yan et al, “100 Gb/s Optical IM-DD Transmission with 10G-Class Devices Enabled by 65 GSamples/s CMOS DAC Core”, thesis No.: OM3H.1, OFC2013.
It should be noted that the above description of the background is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of the present disclosure.